Method of and apparatus for article inspection including speckle reduction

ABSTRACT

A method and apparatus for reducing speckle during inspection of articles used in the manufacture of semiconductor devices, including wafers, masks, photomasks, and reticles. The coherence of a light beam output by a coherent light source, such as a pulsed laser, is reduced by disposing elements in a light path. Examples of such elements include optical fiber bundles; optical light guides; optical gratings; an integrating sphere; and an acousto-optic modulator. These various elements may be combined as desired, such that light beams output by the element combinations have optical path length differences that are greater than a coherence length of the light beam output by the coherent light source.

BACKGROUND OF THE INVENTION

[0001] 1. Field of the Invention

[0002] The present invention relates to inspection of articles, and inparticular, to inspection of articles related to the manufacture ofsemiconductor devices. More specifically, the invention relates to theinspection of articles used in photolithography during the manufactureof semiconductor devices.

[0003] 2. Description of the Related Art

[0004] Current demands for high density and performance associated withultra large scale integration in semiconductor devices require submicronfeatures, increased transistor and circuit speeds, and improvedreliability. Such demands require formation of device features with highprecision and uniformity, which in turn necessitates careful processmonitoring.

[0005] One important process requiring careful inspection isphotolithography, wherein masks or “reticles” are used to transfercircuitry patterns to semiconductor wafers. Typically, the reticles arein the form of patterned chrome over a transparent substrate. A seriesof such reticles are employed to project the patterns onto the wafer ina preset sequence. Each photolithographic reticle includes an intricateset of geometric patterns corresponding to the circuit components to beintegrated onto the wafer. The transfer of the reticle pattern onto thephotoresist layer is performed conventionally by an optical exposuretool such as a scanner or a stepper, which directs light or otherradiation through the reticle to expose the photoresist. The photoresistis thereafter developed to form a photoresist mask, and the underlyingpolysilicon insulators or metal layer is selectively etched inaccordance with the mask to form features such as lines or gates.

[0006] From the above description, it should be appreciated that anydefect on the reticle, such as extra or missing chrome, may transferonto the fabricated wafer in a repeated manner. Thus, any defect on thereticle would drastically reduce the yield of the fabrication line.Therefore, it is of utmost importance to inspect the reticles and detectany defects thereupon. The inspection is generally performed by anoptical system, using transmitted, reflected, or both types ofilluminations. An example of such a system is the RT-8000™ seriesreticle inspection system available from Applied Materials of SantaClara, Calif.

[0007] There are several different known algorithm methods forinspection of reticles. These methods include: “Die to Die” inspection,in which a die is compared to a purportedly identical die on the samereticle; or “Die to Database” inspection, in which data pertaining to agiven die is compared to information in a database, which could be theone from which the reticle was generated. Another inspection methodinvolves Die to golden dye which is a dye chosen as a reference forinspecting wafers. There also is a design rule based inspection, inwhich the dye has to fulfill some line width and spacing requirements,and feature shapes should fit predefined shapes. Examples of theseinspection methods, and relevant apparatus and circuitry forimplementing these methods, are described in various U.S. patents,including, inter alia, U.S. Pat. Nos. 4,805,123; 4,926,489; 5,619,429;and 5,864,394. The disclosures of these patents are incorporated hereinby reference.

[0008] Known inspection techniques typically use imaging the articleswith a large magnification onto a charge-coupled device (CCD) camera.The imaging technique requires the article to be illuminated. Thebrightness of the illuminating source is a key factor in the ability tospeed the inspection by reducing the integration time of the camera. Asthe patterns on wafers get smaller, it becomes necessary to use smallerwavelengths in order to be able to detect the patterns. This is due tothe fact that the physical resolution limit depends linearly on theillumination wavelength and due to interference effects which requirethat the inspection be done at a wavelength similar to the one used inthe lithographic process. As the wavelengths get smaller, conventionalincoherent light sources like filament lamps or gas discharge lamps donot have enough brightness, and the light sources of choice become shortwavelength lasers. The coherence of the laser, together with theroughness and aberrations of the surfaces as well as the patternedarticle along the light path, creates an artifact known as speckle,which is a noisy pattern over the image of the article.

[0009] Speckle causes problems in detection of the surfaces of articlesbeing inspected and causes false alarms because of the non uniformity ofthe light pattern hitting the detector. Detection accuracy is degraded.Also, images taken of inspected articles are degraded. The problem is anacute one in this type of article inspection, because the power providedby coherent light is essential, among other reasons, as a result oflosses stemming from the detection process.

[0010] The just-discussed problems are not unique to inspection ofmasks, photomasks, and reticles. There are known wafer inspectiontechniques which employ coherent illumination. In such systems, specklecan have an adverse impact on yield and performance of the resultingdevices, and so also must be addressed with great care. Examples ofknown wafer inspection systems employing coherent illumination are shownin U.S. Pat. Nos. 5,699,447 and 5,825,482. The disclosures of thesepatents also are incorporated herein by reference.

[0011] When such systems are used to inspect patterned wafers, thespeckle phenomenon can arise, if the spot size used for illumination isnot much smaller than an element of a pattern on the wafer. However, insome circumstances, such as oblique illumination (in which the coherentlight source is directed to the wafer at an angle), the spot size willbe sufficiently large to cause speckle. Reducing the spot size willreduce system throughput and will require working at a wavelength thatis smaller and different from the one used for imaging the article forexample during the lithographic process. Consequently, as can beappreciated, there is a tradeoff between enduring speckle and optimizingdetection sensitivity/throughput. Therefore, it would be desirable tosolve the speckle problem, and thus enable the use of an increased spotsize, and thus improve throughput.

[0012] A comprehensive description of speckle phenomena can be found inT. S. McKechnie, Speckle Reduction, in Topics in Applied Physics, LaserSpeckle and Related Phenomena, 123 (J. C. Dainty ed., 2d ed., 1984)(hereinafter McKechnie). As discussed in the McKechnie article, specklereduction may be achieved through reduction in the temporal coherence orthe spatial coherence of the laser light. There have been variousattempts over the years to reduce or eliminate speckle.

[0013] Another article, citing the above-mentioned McKechnie article andaddressing the same issues, B. Dingel et al., Speckle reduction withvirtual incoherent laser illumination using a modified fiber array,Optik 94, at 132 (1993) (hereinafter Dingel), mentions several knownmethods for reducing speckle based on a time integration basis, as wellas based on statistical ensemble integration. With respect to the timeintegration methods, involving scanning of various planes of the imagingsystem and generating uncorrelated image patterns to be integrated by animage detector, the article identifies some possible drawbacks, such asa long integration time, or introduction of additional optical systemsto support the scanning process.

[0014] Among the methods involving a reduction in the coherence of thebeam, both Dingel and McKechnie discuss the introduction of a dispersingelement, such as a grating, a screen mesh, or a moving diffuser, byitself or in combination with another rotating diffuser, into the pathof the illuminating beam so as to produce a random phase modulation overthe extent of the light beam. Other known techniques involve the passageof a pulsed laser beam through a carbon disulfide cell and furtherthrough an unaligned fiber optic bundle, or the use of liquid crystalsinterposed in the path of the light beam, the crystals being moved byelectric field excitation.

[0015] However, as the reticles become smaller in size and the patternsshrink, it becomes more difficult to manufacture them with no fineanomalies and small defects. With diffraction effects, the detectionbecomes more complicated as well. Therefore, the danger exists thatsmall defects may go undetected, which could cause problems in therelated wafer manufacturing process. One proposed solution to thecurrent situation involves the use of a laser light source emitting lowwavelength laser beams, preferably in the deep ultraviolet (UV) regionto illuminate the article. The laser source would preferably emit shortpulses of light, with a preferred range of 5-50 nanoseconds. None of themethods and systems discussed above is equipped to offer a high level ofspeckle reduction for low wavelength laser beams so as to ensureaccurate defect detection. Also, the above methods and systems do notprovide a reliable solution for short laser pulses because of inadequatemoving speed of the dispersing elements.

[0016] As alluded to previously, speckle also is a known phenomenon inthe wafer inspection area. U.S. Pat. No. 5,264,912, the disclosure ofwhich is incorporated herein by reference, identifies this problem, andprovides some proposed solutions. However, as with other known andproposed speckle reduction techniques, these proposed solutions do notaddress particular problems resulting from the need to work withextremely small features, and the consequent need to employ coherentillumination sources with very low wavelengths.

[0017] Speckle reduction devices, interposed in the light path betweenthe article surface and the detector, also can be expensive. Forexample, interposing a fiber bundle in accordance with one of thetechniques described above could require as many as 10,000 fibers withdifferent properties, such as length, in the bundle. These fiber bundleswould be extremely large in size, and consequently would be expensive.It would be desirable to find a solution that did not need so manyfibers.

[0018] A similar problem would pertain with respect to the use of adiffraction grating. The finer and larger in size the grating, the moreexpensive it would be. It would be desirable to find a solution, usingdiffraction gratings, but which did not require exceedingly finegratings.

[0019] As can be appreciated from the foregoing discussion, there is aneed in the art for a method and system for reducing speckle wheninspecting articles using pulsed laser beams at low wavelengths,including the deep UV region.

SUMMARY OF THE INVENTION

[0020] In view of the foregoing, one feature of the invention is theprovision of an optical system for reducing speckle during inspection ofarticles used in the manufacture of semiconductor devices.

[0021] Another feature of the present invention resides in the provisionof an optical system for reducing speckle in inspection systemsoperating at low wavelengths, particularly in the deep UV region.

[0022] A further feature of the present invention is the reduction ofspeckle in inspection systems using pulsed laser beams, particularlyusing pulses in the 5-50 ns region.

[0023] To provide the foregoing and other features, to overcome thelimitations in the prior art described above, and to solve variousproblems that will become apparent upon reading and understanding of thefollowing detailed description, the present invention provides a methodand apparatus for reducing speckle during the inspection of articlesused in the fabrication of semiconductor devices, especially wafers,masks, photomasks, and reticles.

[0024] In accordance with the present invention, the inventive apparatusis constituted by a coherent light source, such as a laser, whichoutputs a coherent light beam along a light path. In one embodiment ofthe invention, two optical fiber bundles are disposed sequentially alongthe light path, each bundle having a predetermined number of fibers ofdifferent lengths, which may be arranged within the bundle. A firstbundle receives the light beam and outputs several intermediate beams,one beam for each fiber of the bundle, each intermediate beam beingimaged into all of the fibers of a second fiber bundle. Each of thefibers in the second bundle outputs an output beam, which then is usedto illuminate the area under inspection of the article. Various opticaldevices for homogenizing the beam and focusing the beam, as are wellknown in the art, may be interposed at appropriate locations along theoptical path.

[0025] As a variation on the preceding embodiment, the refractiveindices of the fibers within the bundle may be varied. Using varyingrefractive indices may change the optical paths sufficiently to avoidthe need to vary the lengths of the fibers as greatly.

[0026] As a further variation on the preceding embodiment, the fibers inthe bundle may have the same or varying nonlinear characteristics. Oneattribute of employing varying nonlinearity is that the lengths of thefibers may not have to be varied as much. Having fibers of a consistentlength may be advantageous from the standpoint of implementation.

[0027] In accordance with a further embodiment, the apparatus may beusing one or more gratings disposed sequentially along the light path.The gratings operate similarly to the fibers, as discussed above.

[0028] In accordance with further embodiments of the invention, theapparatus may be grating, in either order; or some further combinationof fiber bundle(s) and grating(s), in any desired sequence.

[0029] An advantage of each of the foregoing embodiments is that theindividual elements are either simpler or smaller, and hence lessexpensive, than having a smaller number of larger, or more complexelements. For example, using two fiber bundles with 100 fibers in eachbundle, with each fiber in the first bundle along the path providing anoutput to each fiber in the second bundle, will yield 100×100=10,000different variants, just as if a single bundle with 10,000 differentfibers were used.

[0030] A similar advantage applies with respect to the use of bothgratings which are very good for introducing small optical path lengthvariations, with fibers or light guides, which are convenient inintroducing large optical path length variations.

[0031] In accordance with a further embodiment of the invention, anintegrating sphere, having a first, input aperture and a second, outputaperture, is disposed along the light path, the first aperture receivingthe light beam and with the second aperture outputting a light beam,formed after the light beam has its path changed by being reflectedwithin the integrating sphere. The integrating sphere may be constitutedby two such spheres, with one being disposed concentrically within theother. The inner sphere would provide further reflection of the lightwithin the integrating sphere.

[0032] The just-described embodiment is simpler, in some sense, than thepreviously described embodiments, but has certain disadvantages withrespect to efficiency, based on currently available reflectivematerials. This is particularly so with the two-sphere embodiment. Asthese materials improve, it is expected that the integrating sphereapproach will become an increasingly attractive alternative.

[0033] Yet another embodiment of the present invention is constituted byan electro or acousto-optic modulator disposed along the light path andhaving an input surface for receiving the light beam and an outputsurface for transmitting an incoherent modulated beam to the area to beinspected. The high frequency bandwidth at which the modulator operateswill alter randomly the optical wavefront phase of the input beamsufficiently to reduce or eliminate speckle. This embodiment hassomething of a disadvantage as compared with the previous embodimentsbecause, for the shorter wavelengths such as deep UV, -opticalmodulators are relatively expensive. Since this solution works very wellfor sources with long coherence lengths, it may be used in combinationwith a fiber bundle or a grating, which work well at reducing specklefrom sources with small and medium coherence lengths.

BRIEF DESCRIPTION OF THE DRAWINGS

[0034] The above features and advantages of the present invention willbecome more apparent when referring to the following detaileddescription of preferred embodiments thereof with reference to theattached drawings in which like reference numbers representcorresponding parts throughout, in which:

[0035]FIGS. 1A and 1B show examples of inspection devices for use inaccordance with the present invention;

[0036]FIGS. 2A and 2B show a first embodiment of the present invention,and a variant thereof;

[0037]FIG. 3 is an explanatory diagram related to the first embodiment;

[0038]FIG. 4 shows a variant on the first embodiment;

[0039]FIG. 5 shows a second embodiment of the present invention;

[0040]FIG. 6 shows a variant on the second embodiment;

[0041]FIG. 7 shows a third embodiment of the present invention; and

[0042]FIG. 8 shows a fourth embodiment of the present invention.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

[0043] In the following description of preferred embodiments of thepresent invention, reference is made to the accompanying drawings, whichform a part thereof, and which show by way of illustration specificembodiments of the invention. It is to be understood by those of workingskill in this technological field that other embodiments may beutilized, and structural changes may be made without departing from thescope of the present invention.

[0044]FIG. 1A shows an exemplary inspection device for use in accordancewith the present invention. The inspection device shown in FIG. 1A isoperating in a reflective mode. However, it is to be understood thatinspection devices operating in a transmissive mode, or in both atransmissive and a reflective mode, are within the contemplation of theinvention. At least for the transmissive mode of operation, a beamhomogenizer should be employed prior to the beam's entering a coherencereduction/speckle reduction apparatus.

[0045] In FIG. 1A, an article 1 to be inspected, such as a wafer, amask, a photomask, or a reticle, is positioned on an x-y stage 2, whichmoves the article 1 in two directions. The inspection device includes acoherent light source 4, preferably a laser, located on one side of thearticle 1. The light source 4 may be a continuous wave laser, or may bea pulsed laser, emitting low wavelength laser beams in the UV or deep UVregion. The beams emitted by the light source 4 are directed via anoptical system 6, a beam splitter 8 and an objective lens 10 onto thesurface of the article 1. It should be noted that other means ofdirecting the beams onto the article 1, including other optical pathsdefined by suitable structure, also may be used.

[0046] The light beams hitting the surface of article 1 are reflectedvia a relay lens 18 to an imaging detector 20. The imaging detector 20may be a CCD sensor, which could be a 1 x M sensor, or an N×M area ortime delay integration (TDI) or CCD sensor. The sensor 20 enablesimaging of the article 1, while allowing the stage 2 carrying thearticle 1 to move continuously relative to the light source 4.

[0047] The inspection device shown in FIG. 1 further includes acoherence reduction optical apparatus 30, positioned along the path ofthe light beam. The coherence reduction optical apparatus 30, in itsvarious embodiments as discussed to some extent above, is a centralfocus of the invention. The inspection device also includes an autofocusdevice 22, controlling an objective lens 10 via a beam splitter 24, theobjective lens 10 being also positioned along the path of the lightbeam, and a viewing eyepiece 26 receiving the reflected light beam via abeam splitter 28 for viewing purposes.

[0048] The light source 4 is controlled by a control system 40, whichenergizes the light source 4 to emit the light beams.

[0049] In operation, the light beam emitted by the light source 4through the optical system hits the beam splitter 8 being directedtowards the article 1. The effect of this illumination arrangement isgenerally to deliver normal illumination to the article 1. The lightbeam travels along a light path, being reflected through the coherencereduction optical apparatus 30, the beam splitter 24, and the objectivelens 10, onto the article 1. Then, the light beam is reflected from thearticle 1, being imaged onto the sensor 20 via the relay lens 18.

[0050] The reflected light beam contains information about the patterncontained on the article 1, and also provides information regarding anydefects present in the article 1 and on its surface. The coherent natureof the light source 4, and its wavelength of operation in comparisonwith the size of possible defects, can produce speckle at the sensor 20.Speckle causes unpredicted signal non uniformities, thus making itharder to distinguish the defects, and may allow some microscopicdefects to remain undetected. Therefore, there is a need to reduce thespeckle phenomenon by breaking the coherence of the light beam. Thecoherence reduction optical apparatus 30, positioned along the path ofthe light beam, reducing the coherence of the beam that hits the surfaceof the article 1, thereby reducing or eliminating speckle.

[0051] Another embodiment of an exemplary apparatus is shown in FIG. 1B,relating to wafer inspection systems employing oblique laserillumination. As shown, laser source 4 provides a laser beam thatilluminates wafer 1 at an oblique angle, sometimes referred to as agrazing angle. As a result of the oblique angle, the illumination spot 5has an oval shape, with a major axis of the oval elongated in thedirection of illumination. This spot shape provides a relative largespot area, resulting in speckle.

[0052] In these types of systems, the detection scheme is based on darkfield illumination. That is, the light beam is focused to a spot 5 usingan objective 10. Since the wafer has a mirror-like surface, the lightreflects according to Snell's law, as exemplified in the Figure. Thedark field detectors are placed away from the beam reflection. Someexamples of detector placement are shown as detectors 21, 22, and 23.Any combination of some or all of these detectors can be used. However,it should be appreciated that the speckle phenomenon can affect any orall of these detectors. Consequently, it is necessary to place thecoherence reduction optical apparatus 30 in the illumination path of thelaser beam.

[0053] Several embodiments of the coherence reduction optical apparatus30 are provided in accordance with the present invention. Eachembodiment now will be described in detail.

[0054] According to FIG. 2A, a first embodiment of the coherencereduction optical apparatus 30 includes two optical fiber bundles 301and 302, each having a predetermined number of optical fibers 304, 305.The bundles may have the same number of fibers, or different numbers offibers. There also may be more than two bundles provided. The opticalbundles 301 and 302 are sequentially positioned along the path of theimaging beam I, bundle 301 being positioned ahead of bundle 302 andhaving an input for receiving the imaging beam I and an output adjacentto an input of the second bundle 302. (For ease of description, the twobundles 301, 302 are shown at a slight angle with respect to each other,though in operation they may be aligned along an axis.).

[0055] The optical fibers 304 and 305 are of a similar type known in theart, preferably having predetermined parameters, such as refractiveindex and cladding, although they could also have different refractiveindices or different claddings. The fibers 304 and 305 have differentlengths and may be randomly disposed within each bundle, using any of anumber of known randomization techniques. The difference in length ALbetween any two fibers 304 is preferably selected to be greater than thecoherence length of the light source 4. The difference in length betweenany pair 304 is preferably larger than the difference in length betweenthe shortest and longest fiber in bundle 302. This will maximize theeffect of the combined bundle, provided the right coupling is used. Ageneral relationship among the fibers in the bundle is depicted in FIG.3.

[0056] As also depicted in FIG. 3, one arrangement of the fibers 304within the bundle 301 permits all of the ends of the fibers 304 at theoutput of the bundle 301 to be situated in a plane substantiallytransverse to the path of the imaging beam. Similarly, all ends ofoptical fibers 305 at the output end of bundle 302 may be situated in aplane substantially transverse to the path of the imaging beam. Anotherpossibility is to use a bundle of closely packed light guides withvarying lengths as shown in FIG. 3.

[0057] An example is the case of optical coherence length of 1 mm. Forexample, using a bundle of 25 fibers with lengths from 400 mm inincrements of 20 mm, and a second bundle which consists of 36 closelypacked square light guides with lengths of 20 mm in increments of 1 mm,the effect is as if a bundle of 25×36=900 fibers with length incrementsof 1 mm were used.

[0058] The inputs of bundles 301 and 302 also may be situated in a planetransverse to the path of the imaging beam. In that event, since all ofthe fibers in each bundle have different lengths, some accommodationmust be made for the difference in lengths, as for example by looping orbending some of the fibers in each bundle. Examples of arrangements ofthe fibers 304, 305 within the bundles 301, 302 are shown in FIG. 4.Those skilled in the art will recognize that fiber bundles having fibersof the same length, but with different refractive indices or differentcladdings could be used in lieu of the above preferred arrangements,with similar results.

[0059] Another variant on the optical fiber or light guiding bundleembodiment is that some or all of the fibers in each bundle may havenonlinear optical characteristics. Like the variation in refractiveindex or cladding, or variation in length of the fibers, the varying ofnonlinear optical characteristics serves to vary the path length of thelight passing through the fibers, and thus serves to reduce thecoherence of the beam that is input onto article 1.

[0060] The nonlinear characteristics may be provided by using germaniumdoped glass, for example. Nonlinear characteristics also may be providedin a single light guide or glass rod. The nonlinear material producesself dual scattering at high power densities, thus enabling spectralbroadening to a few hundred GHz, which in turn reduces coherence length,and thus reduces speckle.

[0061] When the wavefront of the imaging beam I hits the input of thefirst bundle 301, it is effectively broken into multiple optical beams,each beam penetrating one fiber 304 within the bundle 301 and travellingthrough the entire length of the fiber 304. At the output of the bundle301, the beam coming out of each fiber 304 is imaged into preferably allfibers 305 of the bundle 302. Therefore, each fiber 305 receives lightfrom all fibers 304. After the fibers 305 transmit the light through theoutput of the bundle 302, the resulting beams have optical path lengthdifferences which all are greater than the coherence length of the lightsource 4.

[0062] One advantage of imaging all of the light from each fiber 304 inbundle 301 into all of the fibers 305 in bundle 302, along with thelength differences among the fibers in the bundles described above, isthat the effect on reduction in coherence will be multiplicative, notadditive. That is, for N fibers in bundle 301 and M fibers in bundle305, the effect on coherence reduction will be as if N x M fibers wereused, not N+M. As a result, for example, for N=M=100, the effectachieved with this technique is as if N×M=10,000 fibers were used,rather than 200. With this approach, it is possible to use far fewerfibers, and have a much less expensive coherence reduction structure.Other arrangements of lengths of fibers are possible, in which theeffective number of outbound optical path lengths will be the product ofthe number of fibers in the bundles used, rather than their sum.

[0063] An advantage of using multiple fibers, rather than light guidesor glass rods, is that the effective length of the coherence reductionoptical apparatus can be very large. For example, by using 100 fibers ina bundle, having an increment of length of 50 mm, an effective length of50 m can be achieved, packed into a box of only 0.5 m. As a result ofapplication of the foregoing technique, coherence of the beam incidentonto article 1 will be reduced, thereby reducing or eliminating speckleat the detector. In a variant on the first embodiment, as shown in FIG.2B, the coherence reduction optical apparatus 30 may include an imaginglens 303 disposed in between the two bundles 301, 302 for imaging thebeam coming out of each fiber 304 into all fibers 305 of the bundle 302.Those skilled in the art will appreciate that alternate embodiments ofthe present invention could include a plurality of optical fiberbundles, disposed sequentially along the path of the imaging beam I, anda plurality of imaging lenses 303 or other suitable optics interspersedamong the bundles. A light scattering element may be introduced betweenthe bundles to homogenize the angles at which the light hits theentrance of the next bundle.

[0064] In a second embodiment of the present invention shown in FIG. 5,the coherence reduction optical apparatus 30 is constituted by anintegrating sphere 311 disposed along the path of the imaging beam I.The integrating sphere 311 includes an entrance aperture 312 and an exitaperture 313. The aperture 312 has preferably a smaller diameter thanthe aperture 313. The aperture 312 is positioned so as to receive theimaging beam 1. The integrating sphere 311 includes a non-absorbinginner surface made out of a non-absorbing material, preferably magnesiumoxide (MgO), for better reflecting the imaging beam I. Other suitablereflective coatings also may be used.

[0065] After the imaging beam I is reflected within the integratingsphere 311, a reflected beam exits the sphere through the aperture 313.More specifically, the beam exiting the aperture 313 is a collection ofreflected beams inside the integration sphere, each beam travelling adifferent distance from other beams. As a result, the collection ofthese beams exiting the aperture 313 is of reduced coherence as comparedwith the original imaging beam I.

[0066] In a variation on the just-described embodiment, as shown in FIG.6, a second, inner sphere 317 is provided, having an outer surface whichmay be provided with the same non-absorbing material as the innersurface of the sphere 311. The inner sphere 317 and the integratingsphere 311 may be concentric, but this is not required. The imaging beamI entering the integrating sphere 311 through the aperture 312 is brokenup into a collection of beams that are reflected by the inner sphere 317and the inner surface of the integrating sphere 311. The resultingreflected beam which is transmitted to the article 1 through theaperture 313 is a collection of the reflected beams, and has reducedcoherence. The effect of providing the inner sphere is to lengthen thereflective path of the light as it goes from entrance aperture 312 toexit aperture 313.

[0067] Various radii of the sphere 311, and diameters of the entranceand exit apertures are possible. Presently a radius of 25 mm for thesphere 311, a 1 mm entrance aperture, and a 5 mm exit aperture arepreferred.

[0068] The embodiments of FIGS. 5 and 6 presently are considered to beless efficient than the optical fiber embodiment, because of lossesattendant to use of the reflective material. The degradation resultingfrom the FIG. 6 embodiment in particular is considered quitedisadvantageous. Providing a buffer of some kind before the entrance tothe single-sphere embodiment of FIG. 5 may work better. It is expectedthat, as reflectivity of possible coatings improves, these embodimentswill become more attractive.

[0069] As shown in FIG. 7, a third embodiment of the coherence reductionoptical apparatus 30 of present invention is constituted by a firstgrating 321 and a second grating 322, positioned along the path of theimaging beam I. The gratings 321, 322 in the embodiment are diffractiongratings, but it is to be understood that reflection gratings could beused with the same results. The gratings 321, 322 are preferablyidentical, having predetermined pitch Λ, wavelength λ, and firstdiffraction order, although gratings with different pitches and otherproperties could be used as well.

[0070] In operation, the imaging beam I hits the surface of grating 321at a predetermined angle θ_(i,) and is diffracted at an angle θ_(o). Ifthe imaging beam I could be expanded to have a diameter D, as seen inFIG. 7, then an optical path difference (OPD) achieved between the twoedges of the imaging beam can be calculated with the formula: OPD=D tanθ_(i)+D sin θ_(o)/cos θ₁. The diffraction relation is sin θ_(o)=λ/Λ−cosθ_(i). The diffracted light beam subsequently hits the second grating322 and is diffracted. The angle of incidence can be the same angle,θ_(i), as before, and the outgoing angle, θ_(o), also can be the same asbefore. Applying the same calculations, the optical path difference ofthe resulting beam is 2·OPD. It can be seen from the above calculationsthat the optical path difference between the light source and thearticle can be broadened by disposing several similar gratings along thepath of the beam I. It is arranged to deflect the light in the samerotational direction, for example clockwise, until OPD_(n)=n·OPD isgreater than the coherence length of the light source. At that moment,the resulting beam will be incoherent and the process will reduce thespeckle phenomenon. The calculations presented above could also beapplied if reflection gratings are being used in an alternate embodimentinstead of the diffraction gratings.

[0071] It should be noted that, as a result of the use of gratings inthe embodiment of FIG. 7, the path of imaging beam I changes. Thearrangement can be such that the entrance and exit beams have the samecross section, even though in between the gratings, the effective crosssection in the meaningful direction will be much larger. The design issuch that the final grating surface is perpendicular to the direction ofpropagation of the light. This enables, for example, the use of anillumination scheme from that point to the article, called Kohlerillumination. Depending on the number of gratings provided, suitableoptics would have to be provided to direct the coherence-reduced beamsuitably onto article 1. Alternatively, the light source 10 could bepositioned appropriately with respect to coherence reduction opticalapparatus 30 so that the beam output from apparatus 30 would be directedproperly onto article 1.

[0072] Instead of using multiple gratings, light can be passed multipletimes through the same grating, using appropriate mirrors or otherreflection/deflection units to redirect the light, for example, along apath similar to that shown in FIG. 7, i.e. redirection at 90° intervals.Redirection at other intervals also is acceptable.

[0073] A fourth embodiment of the coherence reduction optical apparatus30 is shown in FIG. 8. As seen in FIG. 8, the apparatus 30 isconstituted by an acousto-optic modulator 331, positioned along the pathof the imaging beam I, and ahead of an imaging lens 333. Theacousto-optic modulator 331 is coupled to a source of electronic whitenoise 332 supplying a white noise signal with frequencies which may bein the 1-20 GHz range. The imaging beam I hits a relatively largesurface of the modulator 331 at a predetermined angle, the surfacehaving portions with different frequencies in the same 1-20 GHz range.The imaging beam I is then diffracted by these perturbations in themodulator and undergoes changes in direction and wavelength. Therefore,at the output of the modulator 331, the resulting modulated beam hasnon-coherent properties and can be imaged onto the article using theimaging lens 333. The optical arrangement may be similar to the one usedin the system with the grating. Also, an acousto-optics modulator, or anelectro-optic modulator maybe used with a grating, to account for bothlong and short coherence lengths.

[0074] One aspect of this embodiment is the ability to lengthen theduration of pulses received from a pulsed laser so that they exceed thecoherence length of the imaging beam, and allow a more effective way ofusing a rotating diffuser. Assume the pulse length can be stretched from5 ns to 50 ns by using a fiber bundle with fiber length differences upto 20 m. A rotating ground glass 100 mm in diameter, rotating at 30,000RPM for example, may reduce the speckle modulation by a factor of 5.

[0075] Various combinations of the just-described embodiments also arepossible. For example, a fiber bundle could be used with a diffractiongrating, or with an acousto-optic modulator; a diffraction grating couldbe used with an acousto-optic modulator; and so on. The orders in whichthe various components of the coherence reduction optical apparatus 30are placed are not necessarily critical; however, it is appropriate toreiterate the above caveat with respect to the relation of the elementplaced nearest the beam source, and coherence length, in order to securethe multiplicative advantage of the use of smaller fiber bundlesdiscussed earlier.

[0076] While the invention has been described as set forth above withreference to several preferred embodiments, various embodiments withinthe scope and spirit of the invention will be apparent to those ofworking skill in this technological field. For example, while thepreferred embodiment has been described in the context of a reticle usedin semiconductor manufacture, it is within the contemplated scope of theinvention to apply this simple, powerful technique to inspection ofother patterned articles used in semiconductor manufacturing, or tosemiconductor wafer inspection. Indeed, the inventive method andapparatus are applicable equally to inspection of wafers, as well as tomasks, photomasks, reticles, or any other such product used in similarfashion in the manufacture of semiconductor devices, as for example by aphotolithographic process. Hence, so far as the inventive method andapparatus are concerned, the terms “mask,” “photomask,” and “reticle,”and terms defining similar articles, are interchangeable, and should beso understood by those of working skill in this field.

[0077] Moreover, the coherence reduction techniques of the invention arenot limited in applicability to inspection systems, but instead may beused in any semiconductor-related manufacturing operation wherecoherence reduction techniques are needed.

[0078] In view of the foregoing and other modifications which will beapparent to those of working skill in this field, the scope of theinvention described herein should be considered to be limited only bythe appended claims.

What is claimed is:
 1. In the inspection of a patterned article used inthe manufacture of semiconductor devices, a method of reducing speckle,said method comprising: effecting relative movement between anilluminating beam and said patterned article, said illuminating beambeing obtained by modifying a coherent light beam so as to break, atleast partially, its coherence; said modifying of said coherent lightbeam comprising: disposing a plurality of optical fiber bundlessequentially along a light path between a source of said illuminatingbeam and the patterned article, each of said plurality of fiber bundleshaving a predetermined number of fibers, each of said fibers having aninput and an output, each of said fibers of a first bundle of saidplurality of bundles receiving said coherent light beam through saidinput and each of said fibers of a last bundle of said plurality ofbundles transmitting an output beam through said output, such that anoptical path length difference between any two fibers in each bundle isgreater than a coherence length of said coherent light beam.
 2. A methodas claimed in claim 1, wherein said patterned article is selected fromthe group consisting of wafers, masks, photomasks, and reticles.
 3. Amethod as claimed in claim 2, wherein there are first and second opticalfiber bundles, said second optical fiber bundle being said last bundle,the method further comprising providing an output of each of the fibersin said first optical fiber bundle to an input of each of the fibers insaid second optical fiber bundle.
 4. A method as claimed in claim 3,wherein said predetermined number of fibers is different for each ofsaid first and second optical fiber bundles.
 5. A method as claimed inclaim 3, wherein said predetermined number of fibers is the same foreach of said first and second optical fiber bundles.
 6. A method asclaimed in claim 3, wherein at least some of the fibers in said firstand second optical fiber bundles have different lengths.
 7. A method asclaimed in claim 3, wherein a difference in length between any twofibers in at least one of said first and second optical fiber bundles isgreater than a coherence length of said coherent light beam.
 8. A methodas claimed in claim 3, wherein input ends of the fibers in at least oneof said first and second optical fiber bundles are aligned.
 9. A methodas claimed in claim 8, wherein output ends of the fibers in at least oneof said first and second optical fiber bundles are aligned.
 10. A methodas claimed in claim 3, wherein characteristics of at least some of thefibers in said first and second optical fiber bundles are selected fromthe group consisting of nonlinearity, difference in refractive indices,and difference in cladding.
 11. In the inspection of a patterned articleused in the manufacture of semiconductor devices, a method of reducingspeckle, said method comprising: effecting relative movement between anilluminating beam and said patterned article, said illuminating beambeing obtained by modifying a coherent light beam so as to break, atleast partially, its coherence; said modifying of said coherent lightbeam comprising: disposing at least one optical grating along a lightpath between a source of said illuminating beam and said patternedarticle, said optical grating having a predetermined pitch, said opticalgrating receiving said coherent light beam at an input side thereof andtransmitting an output beam through an output side thereof, such that anoptical path length difference of said output beam is greater than acoherence length of said coherent light beam; and disposing a furtheroptical element between said output beam and said article so as tocontribute to breaking of coherence of said coherent light beam.
 12. Amethod as claimed in claim 11, wherein said patterned article isselected from the group consisting of wafers, masks, photomasks, andreticles.
 13. A method as claimed in claim 12, wherein said furtheroptical element comprises a second optical grating, said at least oneoptical grating having a plurality of outputs, the method furthercomprising providing all of said plurality of outputs to an input sideof said second optical grating.
 14. A method as claimed in claim 11,wherein said further optical element comprises a bundle of opticalfibers, said at least one optical grating having a plurality of outputs,the method further comprising providing all of said plurality of outputsto an input side of said bundle of optical fibers.
 15. A method asclaimed in claim 13, wherein said predetermined pitch is the same foreach of said first and second optical gratings.
 16. A method as claimedin claim 11, wherein said further optical element comprises reflectionapparatus for causing said output beam through said at least one opticalgrating at least a second time.
 17. A method as claimed in claim 13,wherein said predetermined pitch in at least one of said first andsecond optical gratings is greater than a coherence length of saidcoherent light beam.
 18. A method as claimed in claim 13, wherein thereare four of said optical gratings, said fourth optical grating being thelast, and wherein all of the outputs of the first, second, and thirdoptical gratings are provided respectively to the input side of saidsecond, third, and fourth optical gratings.
 19. In the inspection of apatterned article used in the manufacture of semiconductor devices, amethod of reducing speckle, said method comprising: effecting relativemovement between an illuminating beam and said patterned article, saidilluminating beam being obtained by modifying a coherent light beam soas to reduce, at least partially, its coherence; said modifying of saidcoherent light beam comprising: disposing an acousto-optic modulator ina light path between a source of said coherent light beam and saidinspected article so as to effect changes in wavelength and direction ofsaid coherent light beam such that optical path length differencesbetween output beams from said acousto-optic modulator are greater thana coherence length of said coherent light beam.
 20. A method as claimedin claim 19, wherein said patterned article is selected from the groupconsisting of wafers, masks, photomasks, and reticles.
 21. A method asclaimed in claim 20, further comprising operating said acousto-opticmodulator at one or more frequencies in a range of from 1 GHz to 20 GHz.22. In the inspection of a patterned article used in the manufacture ofsemiconductor devices, a method of reducing speckle, said methodcomprising: effecting relative movement between an illuminating beam andsaid patterned article, said illuminating beam being obtained bymodifying a coherent light beam so as to reduce, at least partially, itscoherence; said modifying of said coherent light beam comprising:disposing an integrating sphere in a light path between a source of saidinspection beam and said inspected article, said integrating spherecomprising a first sphere of a predetermined diameter and having aninner surface made of a highly reflective material, said integratingsphere further having a first, entrance aperture of a predeterminedfirst diameter and a second, exit aperture of a predetermined seconddiameter, said coherent light beam entering said integrating spherethrough said first aperture and exiting said integrating sphere throughsaid second aperture after being reflected off said inner surface withinsaid integrating sphere, such that beams output from said integratingsphere have optical path length differences greater than a coherencelength of said coherent light beam.
 23. A method as claimed in claim 22,wherein said patterned article is selected from the group consisting ofwafers, masks, photomasks, and reticles.
 24. A method as claimed inclaim 23, wherein said integrating sphere further comprises a second,smaller sphere within said first sphere and having an outer surface madeof a highly reflective material, said coherent light beam entering saidintegrating sphere through said first aperture and exiting saidintegrating sphere through said second aperture after being reflectedoff said inner surface within said first sphere and said outer surfaceon said second sphere.
 25. A method as claimed in claim 23, wherein saidsecond diameter is larger than said first diameter.
 26. A method asclaimed in claim 23, wherein a radius of said first sphere is 25 mm,said second diameter is 5 mm, and said first diameter is 1 mm.
 27. Amethod as claimed in claim 23, wherein said first and second aperturesare disposed at a predetermined angle with respect to each other.
 28. Amethod as claimed in claim 3, wherein said coherent light beam has awavelength in a deep ultraviolet (DUV) range.
 29. A method as claimed inclaim 3, wherein said light beam is obtained from a pulsating laser. 30.A method as claimed in claim 29, wherein said pulsating laser outputslaser pulses having a duration in a range from 5 to 50 nanoseconds. 31.A method as claimed in claim 13, wherein at least one of said first andsecond optical gratings is a reflection grating.
 32. In the inspectionof a patterned article used in the manufacture of semiconductor devices,a method of reducing speckle, said method comprising: effecting relativemovement between an illuminating beam and said patterned article, saidilluminating beam being obtained by modifying a coherent light beam soas to break, at least partially, its coherence; said modifying of saidcoherent light beam comprising: disposing first and second opticalapparatus sequentially along a light path between a source of saidilluminating beam and the patterned article, said first opticalapparatus receiving said coherent light beam and said second opticalapparatus transmitting an output beam that has an optical path lengthdifference greater than a coherence length of said coherent light beam.33. A method as claimed in claim 32, wherein said first opticalapparatus is a fiber bundle having a plurality of optical fibers, andsaid second optical apparatus is an optical grating.
 34. A method asclaimed in claim 32, wherein said first optical apparatus is an opticalgrating, and said second optical apparatus is a fiber bundle having aplurality of optical fibers.
 35. A method as claimed in claim 32,wherein said first optical apparatus is an optical grating, and saidsecond optical apparatus is an acousto-optic modulator.
 36. A method asclaimed in claim 32, wherein said first optical apparatus is a fiberbundle having a plurality of optical fibers, and said second opticalapparatus is an acousto-optic modulator.
 37. A method as claimed inclaim 32, wherein said first optical apparatus is an acousto-opticmodulator, and said second optical apparatus is an optical grating. 38.A method as claimed in claim 32, wherein said first optical apparatus isan acousto-optic modulator, and said second optical apparatus is a fiberbundle having a plurality of optical fibers.